Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

719
FXUS63 KLOT 252339
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
539 PM CST Thu Dec 25 2025

.KEY MESSAGES...

- Cloudy conditions persist, with nearly steady temperatures
  tonight.

- Another period of drizzle and light rain showers likely (70 to
  80% chance) tonight into early Friday morning. A period of
  dense fog may occur, as well.

- Widespread moderate rain with embedded thunderstorms and heavy
  rain possible on Sunday, especially near and southeast of
  I-55.

- A return to more winter-like conditions is on track to arrive
  next week. Wind chills falling toward zero may return to the
  region as early as Monday morning, followed by a return of
  chances for snow by midweek.

&&

.DISCUSSION...
Issued at 238 PM CST Thu Dec 25 2025

Through Friday Night:

It is an unseasonably mild Christmas afternoon across the
region, albeit cloudy. Clouds will hang tight with us tonight
into Friday, as our next weather maker over the Plains quickly
shifts eastward into the lower Great Lakes. Extensive cloud
cover will keep temperatures fairly steady (in the lower 40s
north of I-80 and mid to upper 40s south of I-80) this evening.
Thereafter, readings look likely to climb a couple of degrees
overnight (particularly south of I-80) as the lower-level flow
briefly shifts to the south-southwest in advance of the
approaching weak surface low.

Precipitation chances (in the form of drizzle and light
showers) will ramp-up from west to east across the area later
this evening and overnight as low-level moistening tied to
increasing isentropic upglide fosters lowering cloud heights
and increasing cloud depths. Overall, we are not expecting a lot
of rain from this overnight activity, but some areas across far
northern IL could end up with a couple tenths of an inch before
it shifts east of the area by mid morning on Friday. However, of
more concern is the threat for a period of fog very late
tonight into Friday morning as the surface low tracks directly
overhead. At the very least, very low clouds (less than a
couple hundred feet AGL) will occur with the low overnight into
early Friday morning. However, such environments are also
favorable for dense fog, and given that there is plenty of fog
already noted upstream of our area, I think there is a better
than 50/50 chance that some dense fog will materialize in at
least parts of the area into Friday morning.

The main uncertainty at this point lies with the extent of the
fog into northeastern IL, where ongoing rain showers overnight
could keep visibilities from tanking for several hours.
Accordingly, my western areas (generally interior sections of
IL) look to have the best chance for this period of dense fog
Friday morning. With that said, I cannot rule out some of this
fog working its way into portions of the Chicago metro area for
a few hours around or shortly after daybreak Friday morning. The
current plan is to pass our concerns on to the evening shift
and have them monitor observation trends through the evening to
determine if a Dense Fog Advisory may be needed for parts of
the area.

Cloud cover may be slow to clear on Friday, with much (if not
all of) of the day potentially remaining cloudy again. However,
any dense fog should gradually abate into Friday afternoon. In
spite of a turn towards northwesterly winds following eastward
departure of the surface low on Friday, temperatures will remain
mild for the time of year, with readings generally holding in
the mid to upper 40s north, to the mid and upper 50s far south.

A weak surface ridge axis is progged to shift across our area
Friday night. Light winds under this ridge could set the stage
from some good radiational cooling Friday night, assuming we are
able to scatter out our lower cloud deck by Friday evening. If
this occurs, an additional period of fog could materialize late
Friday night into early Saturday morning.

KJB


Saturday through Thursday:

The forecast in the long term picks up Friday evening when
conditions should be quiet around the area. Following
tonight`s/Friday morning`s rain potential, relative high
pressure will settle in Friday night into Saturday. Forecast
soundings depict enough moisture will remain trapped beneath the
subsidence inversion to provide a setup for areas of fog Friday
night into Saturday morning, especially outside of the interior
Chicago metro. Probs for dense fog from the HREF get pretty
high (widespread >50%) west of the Fox Valley.

During the day on Saturday, a baroclinic zone will gradually
meander northward across southern and central IL east of a
deepening low pressure system. We`ll remain north of the
boundary during the daytime and afternoon highs are progged to
range from the mid 40s in our north to middle to upper 50s in
our south. Before the low tracks across the CWA early Sunday,
the warm front will surge northward into our CWA late Saturday
night. This will bring steady, if not slightly rising,
temperatures during the overnight. Guidance has recently slowed
the progression of the low across the CWA to later in the day on
Sunday, putting a delay on the strong cold advection expected
in its wake. There`s still a fair deal of uncertainty, but
latest guidance suggests this should allow Sunday afternoon
temperatures to warm into the upper 40s and 50s around the area
before they come crashing Sunday night.

As far as precip expectations go with this system, isolated
pockets of rain may be found well out ahead of the low as early
as Saturday evening. Better rain chances will be found
immediately ahead of the low center and especially during the
afternoon. As the associated mid level shortwave works into the
Midwest Saturday night, it will phase with an existing upper low
straddling the US-Canadian border to the north. As it does, the
storm`s warm conveyor will strengthen rapidly into Saturday and
the baroclinic zone along the cold front will sharpen up
significantly; most camps even resolve a secondary low spinning
up along the baroclinic zone upstream. A strong, deep jet will
also develop atop the warm conveyor. As this whole process takes
place, expectations are for precip coverage to expand
significantly, mainly along the face of the sloped cold front
and track of the secondary low. By the time this happens late
Sunday morning and afternoon, the corridor of greatest precip
potential will be focused away from our northwest and more
toward areas near and southeast of I-55 and over into Indiana.
Model camps agree on around or just over 1.5" of PWAT in our
south and southeast by Sunday afternoon. Noteworthy dynamic
forcing will also take place as the trough swings across. 500mb
height falls are in excess of 20dam/12hr during the morning and
afternoon. All signs lead to widespread moderate rain during the
day on Sunday, primarily near and southeast of I-55 where
coverage and intensity look to be generally lesser farther
northwest. Most deterministic camps resolve a pretty sharp QPF
gradient across our CWA with a few tens of miles possibly
meaning a difference of several tenths of an inch of rainfall.
Confidence is rather high that the highest precip totals locally
will be found east of the I-55 corridor. Probs for greater than
1" of precip from the LREF (GEFS/EPS/GEPS conglomerate) are up
to 25% in our southern and southeastern CWA and as high as 50%
for greater than 0.5".

There`s even a potential for a few thunderstorms in our south
on Sunday. Models resolve as many as several hundred Joules of
MUCAPE into our southern and eastern CWA during the late morning
and afternoon owing to some marginally steeper mid level lapse
rates, which could be ample for thunderstorm development. There
will be plenty of layer moisture and forced ascent to make the
most out of this instability. Any storms that do develop will
also have access to upwards of 40kt of effective shear. This
means a potential for pockets of heavy rainfall embedded in
thunderstorms. While the ground is well-thawed by now, soil
moisture remains rather high likely due to the recent snowmelt.
This means some localized flooding or ponding may be possible
beneath any heavy rainfall, but this doesn`t appear to be a
widespread concern. The NBM populated the forecast with slight
chances for thunder near and south of US Rt. 24 which was
maintained in this package.

In the cold advection behind the system, guidance favors a
brief period of a wintry mix or light snow showers to close out
the precip late Sunday. Outright snow showers appear more likely
closer to the lakeshore. No meaningful accumulations, if any,
are expected. One possible exception could be northern Jasper
County in Indiana where some models want to bring in a couple of
inches of lake-effect snow, but the greater potential is
farther northeast.

Another thing of note regarding this system will be strong
northwest winds developing during the latter half of Sunday and
continuing through the day on Monday. Gusts could get up to the
25 to 30 mph range by early evening and possibly closer to, if
not in excess of, 40 mph Monday morning when the strongest winds
are expected. MSLP rises Monday morning will push 1mb/hr. Winds
will gradually ease during the late afternoon and evening.

We can also expect much cooler conditions beginning on Monday
in combo with the strong winds. Morning temperatures are
forecast in the teens and lower 20s with wind chills in single
digits on either side of 0F. Wind chills in the single digits
above 0F are forecast during the warmest part of the day.

Upper 20s and lower 30s are forecast for Tuesday and Wednesday.
Medium range guidance has been struggling with a clipper system
that it wants to bring through the Midwest on New Years Eve
(Wednesday). Most ensemble members favor the upper Midwest and
leave the local area unscathed, but there are a handful of
members, namely within the EPS, that track of swath of
accumulating snow across the area. While the potential for this
appears low at this time, given the holiday, we will keep a
close eye on forecast trends for the possibility of any travel
impacts.

Doom

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 539 PM CST Thu Dec 25 2025

Aviation forecast concerns:

- Gradually lowering MVFR CIGs this evening with some patchy
  drizzle developing late.

- Rain/drizzle/fog develops late tonight with a period of
  LIFR/VLIFR conditions likely predawn into Friday morning.

- Slow improvement in CIGs from IFR to low-MVFR Friday
  afternoon.

- Low confidence in wind direction around daybreak before
  becoming NW by late morning/midday.

Another surface low pressure system was developing late this
afternoon in the vicinity of the mid-Missouri River valley in
association with an approaching mid-level disturbance. The low
is expected to slowly deepen as it tracks east tonight, and
passes across northern IL early Friday morning. Return flow
moisture ahead the low this evening is expected to result in a
period of lowering MVFR cigs, and eventual development of
drizzle/rain and fog with further decay into LIFR/VLIFR
conditions overnight. There is considerable guidance support for
a period of 1/2SM VV002 or even locally lower conditions from
predawn through mid-morning in the vicinity of the surface low.
Overall, have slowed the development in precip and the lowest
flight conditions a bit from previous forecasts. Have remained
somewhat conservative with VLIFR at this point, but could need a
longer prevailing LIFR/VLIFR period with later updates. Precip
ends and visibility should improve with a shift to northwest
winds behind the low midday/afternoon Friday, though current
guidance suggests slow rises in cigs through IFR range.

Wind wise, current easterly winds 10-15 kt will veer southeast
this evening ahead of the deepening low. Models generally take
the center of the low right across northern IL toward morning,
lowing confidence in details of wind direction as the low passes
around daybreak. Confidence is high outside of the actual low
passage however, that winds will be southeast overnight and
northwest behind the low Friday midday/afternoon.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until noon CST Friday for Winthrop Harbor
     to Northerly Is. IL.

     Small Craft Advisory until 6 AM CST Friday for Northerly Is.
     to Calumet Harbor IL.

&&

$$

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